The object of this invention is to provide a means of establishing the position of a vehicle or person moving along a defined track or route and in communication with a fixed point through a radiocommunication system in which the propagation means of the radio signals is guidance by the walls of a tunnel containing the route or track or by a leaky feeder (sometimes called a radiating cable) which follows the same track or route or runs closely parallel to it.
Previously, arrangements have been used or proposed for such purpose whereby the position of the vehicle or person has been determined by an adaptation of normal radar principles in which the elapsed time between an interrogating signal and its response or echo is taken as a measure of the distance involved. Another arrangement, described in the British Pat. No. 1,480,779 and in U.S. Pat. No. 4,041,495, established the position of a vehicle or moving machine by a comparison of the respective phases of a radio signal transmitted from the vehicle or machine and received at the two ends of its track. Such arrangements are difficult to engineer and impose serious constraints on the type and characteristics of any repeaters or amplifiers such as are usually necessary in a system when a leaky feeder is employed as the propagation means over distances of several kilometers or more.
Other arrangements employ short-range wayside radio beacons with associated receivers on the vehicles, or alternatively such beacons on the trains with associated receivers by the wayside, and update stored information relating to the position of the vehicle whenever the train passes the beacon or receiver. Such arrangements are expensive if close accuracy and thus close spacing of the beacons or receivers is required, and also still require a radiocommunication channel if the information is to be relayed between the train and a fixed point. Another disadvantage is that if the stored information is lost through a system fault or power failure it cannot be restored until the vehicle passes a beacon or receiver.